The present invention relates to polyol/polyisocyanate adhesive compositions and more particularly to such compositions having a unique catalyst system which provides the heretofore unknown combination of a relatively long pot life, i.e., open, reaction, working or cure time, and a relatively fast xe2x80x9con demandxe2x80x9d or xe2x80x9ctriggerxe2x80x9d cure.
Reactive urethane-based adhesives have proved suitable for a wide variety of applications including the laminating of structural or decorative panels. Indeed, materials as diverse as coated and treated steel, aluminum, isocyanate foam, fiberglass-reinforced plastics, polystyrene foam, cement board, luan and particleboard may be bonded or joined using urethane adhesives.
Conventional polyurethane adhesives generally are provided as a two-part system having a polyisocyanate base component and hydroxyl curative component which is typically a polyol. Application of such adhesive systems comprehends the mixture of the two components just before being applied to the adherends by tactile methods which include conventional roll coating, hot-nip processing, or meter mix dispensing, or by spray techniques utilizing, for example, a dual feed spray head. However, regardless of the application technique, equipment having plural delivery capability is mandated as the two adhesive components are inherently reactive with an attendant viscosity increase that precludes effective wetting of the adherends.
Although uncatalyzed, two-component urethane adhesives offer the advantage of an extended pot life at ambient temperature, such systems are of little commercial interest as having cure times that are prohibitively long even at elevated cure temperatures. Accordingly, to promote either the room-temperature or elevated-temperature curing of the system, a catalyst, typically an organometallo compound such as dibutyltin dilaurate or an amine or amine-like compound, generally is included with the polyol component. The catalyst must be included with the polyol component as inclusion with the isocyanate component presents the potential for a premature reaction with or gelation of the polyisocyanate. Unfortunately however, although the addition of conventional catalysts can effect the rapid conversion or cure needed for commercial production, catalyzing the adhesive system to such an extent so shortens pot life that the utilization of the aforementioned tactile or spay application techniques is precluded.
In the polyurethane coatings field, aliphatic urethane coating systems having both extended open times and fast xe2x80x9con demandxe2x80x9d or xe2x80x9ctriggerxe2x80x9d cures have been realized by the utilization of mercaptan complexing agents to block the activity of a tin or bismuth catalyst. As described by Linden in U.S. Pat. No. 4,753,825 and by Dammann et al. in U.S. Pat. No. 4,788,083, the disclosures of which are expressly incorporated herein by reference, such complexed catalysts remain essentially inactive until unblocked in the presence of an amine catalyst or heat. Although having a demonstrated commercial advantage in aliphatic urethane coating systems, the mercapto-tin or mercapto-bismuth complexed catalysts have failed to garner commercial acceptance as catalysts for urethane adhesive systems. In part, such failure can be attributed to the fact that urethane coatings typically comprise tin-catalyzed aliphatic polyisocyanates whereas urethane structural adhesives typically comprise aromatic polyisocyanates which generally require an amine or amine-like catalyst for a full cure. Indeed, although amine catalysts such as tertiary amines or the like have been suggested as activators for the above-mentioned mercapto-tin or mercapto-bismuth complexed catalysts, such activators are introduced as a third component. That is, the amine is provided in a vaporous form to which an applied polyisocyanate-polyol film coating is exposed for its curing. It is apparent, however, that such a technique is not amenable for the curing of structural adhesive films which, when applied in a bondline, lack the exposed surface area for a vapor permeation curing to be effective. Including such activators with the polyol component is not a viable option since the activator would prematurely unblock the complexed mercapto-tin or mercapto-bismuth catalyst and deleteriously affect the pot lift of the system when mixed.
In view of the foregoing, it may be seen that despite the improvements made in the curing of urethane coatings, similar improvements still have yet to be realized with respect to the curing of urethane adhesives. Accordingly, there remains a need for a urethane adhesive systems having both long open times and fast xe2x80x9con demandxe2x80x9d or xe2x80x9ctriggerxe2x80x9d curing. Such a need in the art is addressed by the present invention.
The present invention is directed to a novel polyol/polyisocyanate adhesive composition which affords the heretofore unknown combination of long open times and fast xe2x80x9con demandxe2x80x9d or xe2x80x9ctriggerxe2x80x9d cure times. The composition is catalyzed with a catalyst combination which displays a unique and unexpected synergy as neither of the catalyst components alone yields both a long open time and a fast cure time.
It is, therefore, an object of the present invention to provide a curable composition having components which form a catalyzed, heat-cured composition when admixed and heated above a threshold temperature. The composition comprises an isocyanate component and a catalyzed component having a hydroxyl-functional component. The hydroxyl-functional component is catalyzed with a catalyst combination comprising a heat-activatable amine or amine-like catalyst which is activated at temperatures above the threshold temperature, and an activatable complexed metal catalyst comprising the reaction product of a metal catalyst selected from the group consisting of tin catalysts, bismuth catalysts, and mixtures thereof, and a molar excess of a mercapto compound complexing agent.
It is also an object of the present invention to provide a method for curing a curable composition. The method entails providing a curable composition comprising the mixture of an isocyanate component and a catalyzed component. The catalyzed component has a hydroxyl-functional component catalyzed with a catalyst combination comprising a heat-activatable amine or amine-like catalyst which is activated at temperatures above the threshold temperature, and an activatable complexed metal catalyst comprising the reaction product of a metal catalyst selected from the group consisting of tin catalysts, bismuth catalysts, and mixtures thereof, and a molar excess of a mercapto compound complexing agent. The curable composition is heated above the threshold temperature to effect its curing by the activation of the amine or amine-like catalyst and the complexed tin catalyst.
Advantages of the present invention include the ability to formulate a urethane adhesive composition having a long pot life, but which is curable on demand by the introduction of heat. These and other advantages of the present invention will be readily apparent to those skilled in the art based upon the disclosure contained herein.